In order to cool the cooling water of the engine of a work machine, the work machine is provided with a radiator and with a cooling fan that blows air at the radiator. One such type of cooling fan is driven by a hydraulic motor or the like that is driven independently from the engine. And a type of hydraulically driven cooling fan is known that is capable of performing cooling efficiently due to the rotational speed of the cooling fan being changed according to the temperature of the cooling water and according to the engine rotational speed (refer to Patent Document #1).